This invention relates to improved basic polyether polyols, to a process for their production, and to their use for the production of polyurethane and polyisocyanurate plastics.
Alkylene oxide adducts of tolylenediamines ("TDA"), more especially 2,4- and 2,6-TDA and mixtures thereof, and their use for the production of polyurethanes have long been known. E.g., British Patent 972,772. DE-OS 2,017,038 also describes polyethers based on o-TDA (a mixture of 2,3- and 3,4-TDA) and their use for the production of polyurethanes. In these polyethers, all the aminic protons of the o-TDA react with epoxides.
TDA is produced in large quantities as starting material for the production of tolylene diisocyanate ("TDI"). Crude TDA, which contains small amounts of o-TDA in addition to the 2,4- and 2,6-isomers, is readily available and is thus attractive as a starting material for rigid-foam polyethers. For some years, it has been standard practice in the production of TDA to purify the crude TDA by distillation, with o-TDA accumulating as secondary product. This inexpensive by-product is now increasingly being used as a starter for basic polyethers.
Work in the area has intensified in the last 10 years, following reports (U.S. Pat. No. 4,209,609, believed to be equivalent to EP-A 01,800) that rigid polyurethane foams based on certain TDA polyethers show some major advantages over conventional rigid polyurethane foams. Such TDA polyethers have OH values in the range of 400 to 630 and are produced by sequential reaction of TDA with approximately 4 mol (per mol of TDA) of ethylene oxide and then propylene oxide. That is, in addition to a high OH value and a minimum ethylene oxide content, all the terminal groups must be secondary. The advantages include, in particular, an extremely low co-efficient of thermal conductivity, favorable flame resistance, and good toughness.
Pure propylene oxide polyethers produced by conventional methods show extremely high viscosities (&gt;50,000 mPa.s at 25.degree. C.) in the OH-value range mentioned and give rigid foams with only the usual properties. Relatively low-viscosity, pure propylene oxide polyethers of the type produced in accordance with U.S. Pat. No. 4,391,728 (i.e., at high temperatures in the presence of at least 0.8% alkali metal hydroxide) suffer similar disadvantages. Moreover, pure ethylene oxide polyethers, because of their primary OH groups, are too active for most rigid-foam applications.
Polyethers which satisfy the requirements already mentioned, namely approximately 4 mol per mol TDA of ethylene oxide and then propylene oxide, are described in U.S. Pat. No. 4,209,609. According to this patent, the alkoxylation catalyst is preferably added after the ethylene oxide stage. If, however, one of the disclosed amine catalysts, such as triethylamine or pyridine (see Comparison Examples c and h in Table 1), is added before the ethylene oxide, the products obtained are of extremely high viscosity and contain only tertiary amino groups. Where potassium hydroxide is used directly (Comparison Example d in Table 1), the products, although of low viscosity, are not reproducibly obtained (as observed in DE-OS 2,017,038, page 5, line 17) and in addition show a very high content of free o-TDA (1 to 2% by weight).
A two-step process for the production of low-viscosity polyethers based on o-TDA in the OH-value range of 300 to 500 is described in U.S. Pat. No. 4,562,290. In the first step of this process, 1 to 3 mol ethylene oxide per mol o-TDA is added at 125.degree. C. and, in the second step, propylene oxide is added (following addition of potassium hydroxide as catalyst) at a temperature of at least 140.degree. C.
The polyethers based on o-TDA obtainable in accordance with U.S. Pat. No. 4,209,609 are of only limited use. In particular, rigid-foam panels more than 10 cm thick, which are produced in conventional laminator machines, cannot be made with these materials. However, such panels are increasingly being stipulated in the building regulations of many countries. Although, as mentioned above, foams having low coefficients of thermal conductivity, good flame resistance values and toughness values combined with good flow behavior can be obtained using the method of U.S. Pat. No. 4,209,609, such foams show a marked tendency towards cracking and core discoloration, particularly in large layer thicknesses.
It has now surprisingly been found that TDA polyethers showing outstanding foaming properties without the disadvantages mentioned above, namely cracking and core discoloration, can be used to produce foam in thick layers. These TDA polyethers provide the desired property improvements with regard to high flame resistance, good toughness and, in particular, an even further reduced coefficient of thermal conductivity in relation to the prior art. According to the present invention, only o-TDA is used and the sequential alkoxylation with ethylene oxide and then with propylene oxide is carried out from the outset in the presence of special amine catalysts described in detail hereinafter.